When a pump is stopped to stop supplying a fluid, such as water, a check valve installed in a pipeline for conveying the fluid is often unable to deal with a sudden change in the state of the fluid, i.e., normal flow .fwdarw.stop .fwdarw.reverse flow, the check valve closes with a delay, and the valve element of the check valve is seated on the valve seat of the same after the start of the reverse flow of the fluid. Consequently, water hammer is created in a section of the pipeline on the downstream side of the check valve at the moment the check valve is closed, which sometimes cause a serious trouble.
Inventions relating to check valves for preventing such water hammer are disclosed in, for example, JP-40-3654B(Water Hammer Preventing Pumping Apparatus), JP-51-25930B (Improvements in Water Hammer Preventing Pumping Apparatus), and JP-63-60274 B(Swing Type Water Hammer Preventing Check Valve). These inventions will inclusively be designated as "original inventions". As is generally known, those prior art check valves have prevalently been used. The present invention relates to improvements in the water hammer preventing check valves of the original inventions.
The prior art water hammer preventing check valves of the original inventions are based on clear technical ideas surpassing generally known symptomatic technical ideas of preventing water hammer (symptomatic means which allows the valve element to be seated on the valve seat after the start of reverse flow, and retards the operation of the valve element by a damping means or installs an automatic valve and a surge tank in the pipeline to relieve shocks). It can readily be gathered from descriptions in the specifications of the original inventions that the technical ideas on which the original inventions are based include radical improvements intended to stop a reverse flow in a pipeline for the positive elimination of causes of water hammer by forming a valve element and flow passages around the valve element so that the valve element is seated substantially on a valve seat at the moment the inertial flow of a fluid in a discharge direction in the pipeline stops when the conveyance of the fluid is interrupted.
As shown by way of example in FIG. 3, a water hammer preventing check valve of the original invention is constructed so as to eliminate as many factors obstructing a valve closing motion as possible as indicated by solid lines; that is, a valve element is designed so that the valve element has an adjusted shape and a reduced mass, provides the least possible inertial resistance, and is subject to the lowest possible form drag or shape drag. This valve element is capable of performing an appropriate closing motion by its own weight in quick response to the reduction of the flow velocity of the fluid, small errors can be corrected by an additional valve closing force exerted by a weight or a spring, and thereby water hammer can be nearly perfectly prevented.
However, even the water hammer preventing check valve of the original inventions has an unsolved technical problem that "pressure rise" after the valve has closed cannot be prevented.
If the valve closes without delay, the velocity of water (fluid) at a position in front of the valve seat and that of water at a position behind the valve seat at the moment the valve element is seated on the valve seat are approximately zero. Therefore, theoretically, water hammer will not occur. However, a water column on a discharge side is extended by its own elasticity under the influence of resistances exerted by a pump and the check valve, and it sometimes occurs that a pressure wave generated when the water column contracts to its original length is checked by the closed check valve to cause a pressure rise. Since such a phenomenon occurs after the check valve has closed, the water hammer preventing check valve of the original inventions is unable to deal with such a phenomenon. Shocks caused by the pressure rise is not as great as those caused by the water hammer and are practically ignorable in most cases. However, under some pipeline conditions, particularly when the length of the pipeline on the discharge side is long, the adverse influence of the shocks caused by the pressure rise can be serious.
On the other hand, prior art slow-closing check valves shown in FIGS. 4 and 5 by way of example are known to be capable of preventing such a pressure rise. Particularly, a slow-closing bypass check valve which is provided with a bypass valve element controlled by a dashpot has prevalently been used.
However, the prior art slow-closing bypass check valve has the following problems.
The slow-closing bypass check valve employs a general check valve, which is liable to delay in valve closing action, as a main valve. When the main valve delays in valve closing action, a main valve element is seated on a valve seat after the flow has reversed. Therefore, measures, such as the employment of a double valve element and the reinforcement of the valve shaft and the associated parts, must be taken to prevent the water hammer prior to the prevention of the pressure rise. Moreover, the bypass valve must unavoidably be formed in a mechanism which permits the reverse flow of a large amount of the fluid and closes in a considerable time to relieve shocks exerted by the water hammer. Accordingly, the bypass valve must be formed in a predetermined bore diameter generally equal to about 1/3 to about 1/4 of the bore diameter of the main valve. Consequently, many design consideration are inevitably necessary for the reinforcement of the component parts and the dashpot, and the scale and complexity of the peripheral devices and mechanisms associated with the bypass valve exceeds those of the bypass valve element which is a principal functional part, so that the check valve has a complex construction. Generally, the bypass valve is mounted on a main valve casing because the bypass valve takes up a considerably large space. However, a large check valve of such a construction has a great height and requires a troublesome work for maintenance, inspection and the adjustment of the dashpot.
Originally, the check valve is used to prevent the reverse flow of a fluid once conveyed forward by energy. Therefore the permission of the reverse flow of a large quantity of the fluid even for shock relieving is quite wasteful and conflicts with the original purpose of the check valve. Accordingly, the quantity of the fluid allowed to reverse must be limited to the least possible extent which will not cause water hammer and pressure rise. Although the closing speed of the bypass valve element of the prior art slow-closing bypass check valve can be adjusted by adjusting the needle valve of the appended dashpot, it is difficult to limit the reverse flow of the fluid by adjusting the size of the bypass passage at the start of the reverse flow (the starting position of the bypass valve element) because the limitation of the reverse flow of the fluid may possibly obstruct the operation of the bypass valve element supposed to allow the reverse flow of a large quantity of the fluid.
Accordingly, it is an object of the present invention to provide an ideal, economically advantageous, high-performance water hammer preventing check valve capable of solving all the problems in the prior art radically, based on a technical idea developed by incorporating improvements into the technical idea of exactly preventing water hammer by controlling a valve element for a closing motion in quick response to the reduction of the velocity of a fluid, on which the water hammer preventing check valve of the original inventions is based, making the best use of the original inventions' advantages for a main valve, provided with a small bypass valve permitting the reverse flow of only a small quantity of a fluid, capable of preventing both water hammer and pressure rise under any pipeline conditions, having a mechanism facilitating the adjustment of the quantity of the reverse flow, provided with a compact main valve and a compact bypass valve, having a reasonable construction not subject to excessive forces, and capable of being easily designed and manufactured.